CometWatch February – part 1

This month, Rosetta is approaching Comet 67P/Churyumov-Gerasimenko at 40 km or less, returning beautiful views of the nucleus and its surface features. In today’s CometWatch image, we see the comet pictured by Rosetta’s NAVCAM on 10 February 2016, when the spacecraft was 50.6 km from the comet nucleus.

In this orientation, the small comet lobe is in the foreground, towards the top left of the frame, and the large lobe is farther away, in the lower-right part of the image. The view reveals most of the comet’s southern hemisphere, which has been experience a short and intense summer since May 2015.

The illuminated portion of the large lobe is dominated by the southern region of Anhur, with hints of Sobek on the neck. Smooth portions of Aker and Khepry are also visible towards the upper edge.

A number of regions are depicted in this view of the small lobe: Maftet, Nut and Serqet towards the lower left, Bastet on the upper right edge of the lobe, but most notably the vast, round cavity of Hatmehit in the top left and the seemingly flat terrains of Wosret at the centre of the image.

The contrast between Hatmehit, covered in dust and boulders, and the rough features on the neighbouring Wosret region were also captured in another striking image, taken with the narrow-angle camera of Rosetta’s OSIRIS imaging system on 13 February somewhat closer to the comet, at 45.8 km.

OSIRIS narrow-angle camera image taken on 13 February 2016, when Rosetta was 45.8 km from the comet. The scale is 0.82 m/pixel and the image measures 1.7 km across. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The OSIRIS image provides a zoomed-in, detailed view onto this portion of the comet as seen in the NAVCAM image, although with a slightly different viewing angle, revealing a great deal of details about these two regions. A portion of Bastet is also visible in the OSIRIS image, including the brighter slab on the top edge of the lobe.

The narrow angle OSIRIS view offers a beautiful view of Abydos right in the centre of frame, partly in shadow. However, I am sure there is a telltale glint from Philae in precisely the location other OSIRIS images had the primary candidate located! I am so glad Rosetta and OSIRIS have allowed the process of discovery in these images (belatedly in respect to OSIRIS) by armchair scientists, citizen scientists and the like. Thank you Claudia, Emily and the whole outreach team!

Marco: No, I don’t think so, I think this is Wosret. It would be good if the Osiris team put in some identifying commentary for their OPOD. Today’s picture (taken by NAC on 13 Feb) appears to be looking down from Bastet onto Aker on the bigger lobe, but it’s so easy to be mistaken.

Hi Marco,
what do you think about 67P as kind of a “cosmic popcorn” as a way to provide a mechanism for your presumed stretch to explain the bilobic/irregular shape of 67P?

“Stretch” isn’t an entire taboo for planetary science, as you might have read in a recent New Horizons post (http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20160218) : “Charon’s surface fractured as it stretched”.
Charon likely expanded when its water froze.
Alan Stern, the PI of the New Horizons mission is also PI of Rosetta’s ALICE instrument. So we can assume, that a consideration of stretch is basically present in the Rosetta science team.

An expansion of 67P might have been driven by gradual sublimation of supervolatiles like N2, O2, CO deep in its interior during warming over millions of years.

So you get “stretch” almost entirely based on observed data and established comet models, with only few ad-hoc assumptions.

You may have probably realised that I’m not such a big fan of your “cosmic popcorn” ideas if I understand them correctly. All the recently “stretch peer” reviewed signatures for stretch (Ok, it is mainly me and A..Cooper reviewing each other’s blogs on stretch) point more and more to centrifugal and other rotational effects as the force driver for fracture and stretch rather than explosive popcorn like expansion of ice and supervolatiles.

Moreover, there appears to be sliding of flexible layers that then solidify on the surface, leaving evidence of their slide frozen in time.

I think the Rosetta teams may just be warming up to stretch – Bibring, as an example, appeared warm to it as early as a year ago, if his Native language 67P seminars were anything to go by.

Perhaps there is now overwhelming evidence in its favour – There may even be hints that stretch is continuing measurably even now!

I’m sure one can come up with stretch with very few ad hoc assumptions, but not consistent with all the recent sliding matches.

That is to say – It needs more than just a few ad hoc assumptions. The requirement is to ignore the frozen considerations of the narrative and look at the evidence without the baggage of needing to fit pristine ingredients.

Hi Marco,
I still see the current state of your elaboration of the stretch hypothesis as fringe.

But this doesn’t necessarily apply to all individual ideas.

I’d think, that short-term spin-ups would either result in some minor mass-loss at the surface, or to disruption and escape of large parts of the comet.

I’d expect either a an initial MacLaurin or a Jacobi ellipsoid shape of the early nucleus due to the rotation of the cloud accreting to the comet.

By self-gravitation and spin-down, this might have grown more spherical over time, provided the supervolatile ices have been sufficiently ductile. It might also have triggered mass-wasting events (land slides).
Spin-up by YORP, asymmetric loss of volatiles or later collisions might have inverted this spin-down process.

I’can’t rule out, that this way a rubble-pile model can eventually result in a contact-binary.

A (long-term) liquid model might result in a pear-shape due to spin-up.

The composition of the evolving gasses strongly suggests, that the core of the nucleus has been very cold all the time since condensation and accretion.

But what happens to supervolatiles which start sublimating between 20 and 30K (like neon) as the core is gradually warmed by an aging Sun, or by an orbit growing closer to the Sun.
What happens to helium released by radioactive decay.
Will there be a pressure build-up deep in the interior of the comet, resulting in an expansion/stretch?
Is the observed porosity a limiting porosity caused by interior release of supervolatile gasses and helium within an initially less porous matrix, stopping expansion when porosity allows to release the gasses? Or has porosity initially been to high to allow for this mechanism?

This is largely independent of the surface erosion/sublimation of the recent few centuries.

I’ve no “faith” in any prejudiced opinions or rhetoric attempts to sell something. I’m just looking at the data, physics, statistics and logic. This results in rough assessments about the likelihood of the scenarios.
Previously I hadn’t considered an expansion scenario driven by gradual release of highly volatile gasses in the deep interior of the comet.

Hi Gerald,
That’s fine, and I think A. Cooper has elaborated on how the hypothesised cold interior could plausibly still be consistent with the stretch matches. We do not parrot eachother’s views on this, and my views diverge with his. We are looking critically at any observations that can shed light on the interior temperature. Chemical signatures related to the outbursts are one way. Another is to consider the effect of Deep Impact probe. Looked at together, Deep Impact went deeper and resulted in days of extra outgassing. 67P outbursts are transient and can be powerful. Looked at together, these indicate a pressurised interior (for Deep impact puncturing a “balloon”) and/or a pressurised liquid bubble (for the 67P outbursts to be both powerful and limited in time scale)

So the circumstantial evidences point to pockets of pressure of various sizes that could include liquids, and the chemical signature is consistent with liquid water mud BLEVEs. The complication is with O2. The only way to reconcile the observed O2 with liquid water is voracious and exotic microbial activity.

This diverges substantially with everything we think we know about life in the universe, without being inconsistent with observations or physics. A.Cooper is very hesitant to look at this possibility, and I want to follow where this takes the science. I am certainly not trying to “sell” the idea. If I was, I would be a terrible salesperson.

A challenge to the consensus of cometary origins is also a challenge to the consensus on life origins.

Hi Marco,
so it appears, that I’m a little closer to A.Cooper’s view regarding the interior of the comet.

Regarding the origin of life: Years ago it has been considered, that pre-biotic metabolism might be possible at very low temperatures, below -200°C. As long as our understanding of the origin of life is as sparse as it is, we should stay open-minded about the state to which pre-biotic organic chemistry can evolve under outer space conditions.
Some organisms from Earth could certainly survive in a dormant state for quite some time in the interior of the comet. But for active biology conditions are very harsh on and in the comet.
Finding out, to which state organic chemistry was able to develop in interstellar dust, and in the interior of the comet is certainly a topic of ongoing research.
And we don’t know pretty much about the organic chemistry of the circum-stellar cloud, which might have contributed to the dust portion the comet is made of.

Gerald,
I have registered on Arxiv to post a paper on stretch theory. However, I need an endorsement from someone in the category I would like to post, as a first time Arxiv author. The appropriate category appears to be Astrophysics – sub category planetary and earth science.

If you or anyone you know may be happy to endorse me so I can progress your suggestion of submitting a paper, I would be very grateful.http://livingcomet.blogspot.com.au/
Is essentially the draft of the paper I am looking to co-write with A.Cooper.

More like expanding insulation foam, I think. The comet has all the ingredients (even certain catalysts though probably not in the correct proportions or distributions). I think expansion may apply to the neck region. Ramcomet called it an expanding muffin mix, which is a good visualisation tool. It may have happened inside the lobes too, but the neck is probably the best candidate.

I think the gradual sublimation over millions of years just won’t happen at the temperatures assumed to be in the interior (circa 30K). So that suggests that, based on Charon, you might be questioning that low core temperature? It seems you might entertain the idea of upping the temperature through primordial heat and radioactive decay. However, I think it’s difficult to argue that for 67P due to the surface area/ volume issue.

But I reckon Marco is on the right track in questioning those assumed core temperatures and citing some kind of slow convection under a sealed, lightly pressurised crust to raise them. It wouldn’t have to go right to the centre to induce expansion/stretch. The core could remain solid and the result would be an avocado deforming and stretching around its stone. It would probably need to be ductile for about 600 metres depth to achieve that. That’s because the amount of the hypothesised stretch (prior to head lobe shear) seems to involve about that much depth.

The above assumes expansion purely based on slow sublimation and expansion of the matrix via that process. However, if you treat it as an adjunct to the stretch event, the stretch itself induces runaway popcorn production. Marco’s higher temps would be harbouring liquid water or hydrocarbons which would BLEVE on sudden exposure to the vacuum or near vacuum in the suddenly depressurising matrix. You seemed quite amenable to BLEVEing last year.

BLEVE’s would expand the matrix before the gases escaped because the event would be very sudden. The matrix would be a fizzing, low-density foam. That would explain the appearance of Anuket and Bastet’s neck region, which in stretch theory, were yanked out of the body lobe in a very short time as the head rose after shearing.

That would be as opposed to Hathor, which stretched more slowly and cleaved to its base prior to rising with the head. If the core really was made of marbles (the expected, modelled size of the ‘dinosaur egg’ constituents) then yes, I suppose you would have popcorn after all. Anuket looks like a mountain of popcorn with a few choc chips sticking out that got pulled up in delaminated lines from the body (like ticker tape).

You said, “So we can assume, that a consideration of stretch is basically present in the Rosetta science team.”

Well, maybe it’s been present for the last few days as a result of this finding but it certainly wasn’t present last October:

This linked Rosetta blog post intimated pretty clearly that stretch had never been on the agenda and would continue to be off the agenda despite the idea being thrown out there.

And perhaps the Charon findings won’t change anything because the crucial difference is the source of the heat to bring about the expansion: primordial heat and radioactive decay. As I said above, it’s difficult to argue that for 67P.

I meant to mention that a lot of my thinking on the above skirts very close to what Robin Sherman has contributed, especially his thoughts on the analogy to ‘firn’ (semi-compacted ice) in the very early days. His comment reply to me is here, with a description of firn and a link to a very interesting phase diagram. That, along with his cryovolcanism, helped immensely in understanding what might be happening in the interior. The 80% porosity suggested a firn-like interior could be present and evidence of possible slurry shows both Robin and Marco could be on the right track. I’m not suggesting Robin is an adherent of stretch theory by the way.

The geometry of firn (or even semi-compacted ice marbles) lends itself to being lubricated by a sudden influx of gases between the interstices whether from endogenous sublimation or a nearby BLEVEing liquid reservoir. If you add in the inbuilt tensile stress via rotational spin-up, any sufficient weakening of the matrix via such lubrication would allow stretch.

Whether firn or marbles, the idea is that only a tiny percentage of the matrix has to become ductile: just the sintering between the loosely compacted constituents. Let’s say the mass of the sintering breakages is 5% of the matrix mass for argument’s sake. This means that an almost entirely brittle matrix becomes ductile. 95% of the brittleness is irrelevant because it’s inside the rolling marbles.

I suppose I should head off any objections that the gas pressures won’t be enough to stretch the comet. It’s a lot more subtle than that. Whilst agreeing with Gerald on the simple, slow popcorn version (without stretch-induced BLEVEs) I think fizzing/ popcorn production alone could induce expansion only nearer to the surface. Further down, there would be too much weight (mass x acceleration due to gravity) countering the gas pressure. Not to mention the immediate pressure reduction via localised dissipation through the porous matrix.

However, there would be a depth at which the pressure overcame the weight. It may actually be quite a shallow depth. That was what I was visualising with Ramcomet’s muffin mix at Anuket (which is the same idea as a fizzing matrix of expanding popcorn). The weight issue means it would only work well (if at all) when a moderately shallow volume of matrix is exposed to the surface. So that surface volume does indeed expand and contribute to what one would call stretch but it wouldn’t be on a large scale across the entire comet and through its core.

But the situation is more complex when spin-up is invoked. As the comet spins up, the surface becomes weightless. Then as it spins up more, the interior matrix at a certain buried radius becomes weightless while the mass above it, including the surface, goes into negative g. That upper portion starts exerting tension on the core. It does so from above the circle/cylinder defined by the weightless radius. This means that at relatively high spin rates, the popcorn fizzing process could conceivably exert pressure at a deeper depth than at low spin-up speeds- i.e. from a point a little below the weightlessness radius for that spin-up rate.

So the fizzing would be pretty feeble as an instrument of comet-wide stretch if the rotation rate was slow. But if the rotation rate was high, fizzing would help in two ways:

1) by lubricating the marbles/firn as described in my comment above. That would occur at all depths, if gases happened to be liberated at those depths, but the pressures would be too feeble to cause stretch below a certain radius from the rotation axis.

2) by actually causing stretch above that critical radius, as well as lubricating marbles above that radius.

This second avenue means that all the matrix above the weightless radius could potentially stretch evenly due to popcorn fizzing alone (which is in addition to the exponential stretch related to radius and due to spin). If the spin rate of the hypothesised single body was 2 hours, a significant depth of the crust would be in negative g and susceptible to further stretch via popcorn fizzing. That depth is around 600-metres for the supposed original body based on my calcs for head lobe detachment (but with rough inputs). So a 600-metre depth would be susceptible to remodelling via fizz at the required 2-hour rotation rate for head lobe departure. And the onion layers go to at least 600 metres.

So the gas pressure inducing stretch argument is a subtle one and it’s spin-dependent. However, stretch theory doesn’t rely on fizzing/popcorn/muffin expansion. Tensile stress along the long axis is the key and possible gas lubrication is only implied by the fact it appeared not to be brittle. Similarly, surface fizzing is implied by sudden gas release at or near to the shear event but isn’t required.

“Further down, there would be too much weight (mass x acceleration due to gravity) countering the gas pressure. ”

An easy sum.

On Earth, under water, density one, you get 1 bar every ten metres. Easy to derive, 10m/s^2 and 1000kg/m^3.

Now if we treat 67P as liquid (which will over estimate the pressure) with a density of about 500kg/m^3 and a gravitational field of around 10^-4 of Earth, on 67P you would get 5*10^-5 of that.
So on earth say 1km, 100 bar
On 69P, 5*10^-3 bar – which on earth would be called a rough vacuum.
Now in fact this will be a drastic overestimate. On Earth, over 1km, the gravitational field is pretty constant; but over 1km on 67 P it will reduce a lot, that’s a good fraction of the radius.
The, pressures are very, very small.

I don’t consider radiogenic heat as relevant for 67P.
Instead, I wouldn’t rule out, that either orbital changes and/or brightening of the Sun might eventually have incremented the core temperature of 67P above the temperature at the time of its accretion.

However, radioactive decay early after accretion could have released helium, as another gas with sufficient pressure at low temperatures. But helium atoms are small, and more likely than neon to escape.

I don’t think, that BLEVEs on the basis of pure water ice would be very efficient near 0°C or below. But related effects e.g. with appropriate clathrates might be possible.

Rethinkig helium: It’s a species in natural gas. due to radioactive decay. So I’m not quite sure, whether all of it would escape from the interior of the comet. Depends on the actual structure of the porosity.

If it doesn’t escape some pressure would have built up early in the history of our solar system, since short-lived isotopes had been much more abundant than today.

Helium tends to diffuse very rapidly through some pretty surprisingly solid materials. Early He-Ne lasers lost helium because it diffused through the soft glass envelopes. I’ve had trouble with helium diffusion across o-rings and through PTFE seals.
Of course diffusion slows at low temperature.
It is retained in oil reservoirs, but I’m dubious most credible low density comet structures would retain it for long.
It will escape thermally once it reaches the surface.

Good eye, Dave, I submitted grinding in the neck as a heat source about a year back. Now Gerald and A. Cooper, Marco are speaking of popcorn and insulation foam type expandimg especially in the neck. It goes back to the expanding neck as the Ramcomet Muffin Top rises up… Rocking lobes, grinding, heating and rising expansion in the neck as the comet stretches.
Logan and others have insights or dabbles on the exact mechanisms, I am just a Daydreamer. 😉

I used to think the temperature was temporarily elevated by hypothesised stretching but as far as I can tell, it would be negligible. I did a rough calculation of the maximum energy expenditure and temperature rise via stretch. This would generate a lot more heat than crust and lid sliding but it’s still a very small amount. I assumed a 5.4 km-square cross section that was stretched and a stretch length of 1km.

You don’t have to crunch the millions of square metres and billions of cubic metres to get an answer. You can do it per cubic metre. If we assume the 5.4 square km stretched by 1km then you could divide it into 1000 slabs that are 1 metre thick and 5.4 km square km. If it stretched by one km, then that’s the same as saying that each slab stretched from one metre to two metres thick. That’s very simplistic because it would pinch in the middle (as the body lobe appears to have done, in my view). But it’s a start

By using the estimated tensile strength value of 40 pascals, you can determine the force needed to overcome that tensile strength and start stretching the comet. 40 pa has been cited as an upper limit for 67P. I wondered if it was just for the crust but I read a paper saying other comets were around the same or a bit more and they implied that it was simply the tensile strength of the bulk material, not just the crust. ianw16 or Gerald might want to weigh in here because they know more about other comets. But even if you times it by ten, the temperature increase will still be very small.

So if each 1-metre-thick slab of 5.4 square km gets stretched to 2 metres thick, each cubic metre gets stretched to 2 cubic metres. That means a tensile force of 40 pa operates on the cubic metre to stretch it through 1 metre. 40 pa is 40 newtons per square metre. We are dealing with a one square metre face to pull on so 40 newtons are applied to that face and pull it through 1 linear metre. That stretches the the cube into a cuboid of 1 x 1 x 2 metres.

The energy expended is the same as the work done. Work is force x distance so the work is 40 x 1= 40 joules.

The only mass that was affected by this operation and energy expenditure is the original 1-metre cube. That has a mass of 533kg and all the energy is dissipated as heat. So the 40 joules goes into heating 533kg. So the heat energy going into each 1 kg of stretching comet matrix is 40/533= 0.075 joules per kg….which is next to nothing.

The specific heat capacity of ice at -100°C is 1389 J/kg°C (joules per kilogram centigrade; -100°C value was lowest ice temperature value in the table). For silty mud it’s about 2500 J/kg°C. If it was made of asphalt, clay or concrete it would be around 1000 J/kg°C. So let’s say the average is way lower than any of these to be safe, say 250 J/kg°C. So 250 joules would be needed to raise the temperature of 1kg of comet by 1 °C. But we only have 0.075 joules per kg. So the temperature would go up by 0.075/250= 0.0003 °C.

The only way this might have a slight effect is if the comet is made of lightly sintered marbles and it’s just the sintered joins offering the tensile strength. When they break, all the expended energy goes into the broken joins. If they were 1% of the matrix mass then you could times the temperature rise by 100. But it’s still small and a highly idealised scenario.

That’s why I think that, if we really are seeing liquid/slurry signatures on the surface, then Marco is on the right track with a lightly pressurised interior, higher core temperature and liquids that can emerge explosively via BLEVEing. This fits with the matrix being more ductile to allow stretch and the observed explosive jets- in addition to the suggested (by us) slurry signatures.

Claudia: Since we are approaching a 30 km distance to the nucleus, a question: will Rosetta be put in a gravitationally bound orbit again (as Accomazzo and Lodiot explained about 17 months ago) ? If so, when will that happen?

In an earlier post the declining activity of the comet was mentioned. Even six months before perihelion, in February last year, we saw the spectacular jets at the neck of the comet, highlighted by the enhancement you did of the pictures. Now we are six months after perihelion at around the same distance from the Sun.

1) Is the activity much lesser than at the same distance last year?

2) Is there no region with activity comparable to that at the neck then?

Rosetta and near-Earth observations both indicate that the activity is reasonably symmetric around a point in time about 2 weeks after perihelion. However, this refers to the global activity, and not to specific components and regions, which is part of ongoing study of the data.

The Southern hemisphere of the comet provided some of the spectacular activity around perihelion, and scientists are examining differences in the overall comet activity as part of the long-term Rosetta science programme.

As for the ion tail, it might be possible to image it from Earth, although the best conditions to observe the comet are over.

Claudia, I agree that the global activity is still pretty spectacular, we saw that in the Twin tails post recently. Inaccurately speaking, this is caused mostly by the dust emitted till perihelion or a little later. It appears the activity on the nucleus itself at this point of time is less, perhaps because the southern hemisphere has already been cleaned up at previous perihelia. So it may be that the Osiris team’s prediction of 20 metres of surface being removed will hold only in places where there was spectacular action around perihelion, and not uniformly over the nucleus.

Ok, I know this will elicit a collective groan and no doubt accusations of just being illicitly dense, but, after looking back through many many photos of P67, the one thing that continues to be the most striking to me is that in literally every picture, no matter how close up or far away, every feature on the comet’s surface still looks either EXACTLY like dust, or EXACTLY like very hard rock (have I mentioned this before?). I just have to wonder how you evenly mix ice/frozen volatiles with various dusts to come up with pretty much all the features of the comet everywhere looking exactly like hard rock, with all those jagged edges, huge cliffs (that seem to be crumbling like rocks), jagged peaks, chips, very long straight ledges, striations, stratifications, etc. etc – in other words, all the things we associate with typical rock formations. Would love to see this replicated. My suspicion is that it couldn’t be, though it’s obviously assumed that this is what has happened – all the processes (mainly sublimation) combined with all the constituents of the comet to cause it to oddly enough look like hard rock everywhere. There’s an absolutely amazing consistency to the rocky look, and the exact features that we all associate with rock formations, boulders and gravel, all caused by dust, ice, and sublimation. In fact, it’s amazing how this comet looks exactly like many rocky asteroids, which IS rock, yet P67 can without any doubt at all only be dust and ice and some unknown rock look-alike surface substance. I mean, exactly what IS this magical surface substance?

So you look at a picture of an asteroid, and then look at a picture of P67, they look exactly alike, but are actually nothing alike at all? Either a mind blowing coincidence, or a mind blowing disconnect. In this case, I guess I still distrust the oft asserted scientific “certainties” over my own eyes (certainties that still don’t actually seem all that certain to me). So my spidey sense goes with disconnect, at least until an actual sample says I’m wrong.

So, which comes first, the chicken or the egg? In the sublimation model, the “rock” is derived from dust. Other option, the dust is derived from rock.

Take a look at pictures of the Kumbu ice fall below Everest, (Ive been there) – but make them black & white.
There are many pictures on the web.

You will see “………with all those jagged edges, huge cliffs (that seem to be crumbling like rocks), jagged peaks, chips, very long straight ledges, striations, stratifications, etc. etc”
But its ice, in that case rather pure ice.

I do *not* mean to imply any similarity whatever in processes or materials at 67P & the Kumbu ice fall! Merely to point out that appearance is of little use.

Sovereign Slave,
I share your exitement, but I must contradict the assessment, that 67P looks like an asteroid. Asteroids are overly covered with impact cratering. You can even estimate the age of larger craters by their smoothing/degradation due to impacts of small meteorites.

This impact cratering is almost completely missing on 67P. This very strongly indicates substantial loss of surface material.
Therefore the fresh-looking surface like outcrops on Earth, where impact cratering is rare, but erosive processes are abundant.
There are different erosion processes at work on 67P than on Earth. But in both cases the result is exposure of “fresh” material at the surface.

Regarding similarity to rock: Water ice is much harder at very low temperatures than near 0°C. This allows for sharp edges and distinct features staying stable for long enough, until new features form by outbursts, fracturing, subsurface sublimation, “rock”falls, slides, etc.
Low gravity reduces the odds for “hills” and cliffs collapsing under their own weight, as long as the matrix isn’t too fractured.

Shouldn’t we see very large differences between the early photos and more recent ones? Differences large enough to wipe out traces of large craters? Or is all cratering and erosion sub-meter detail which we can’t see? Comet looks pretty unchanged to me.

There are many instruments on the Rosetta spacecraft. They’ve detected all sorts of things.
Something those instruments have failed to detect is rock. It is conspicuous by its absence.
This spacecraft has already visited two asteroids. Their density is what one would expect from rock, i.e. greater than ~2000 kg/m^3. Neither of them was sublimating vapour of any description. The thermal inertia of rock is very different from dust and ice. Rosetta measured this at asteroid Steins as being ~650 J m−2 K−1 s^−1/2. The value at 67P is somewhere between 0 J m−2 K−1 s^−1/2 (ice), and ~ 10-60 J m−2 K−1 s^−1/2 (dust).
And asteroids, for the most part, do not look particularly like comets, such as this one: http://i.cbc.ca/1.1595578.1379059437!/httpImage/image.jpg_gen/derivatives/16x9_620/hi-asteroid-852.jpg
or this one: http://annesastronomynews.com/wp-content/uploads/2012/02/Asteroid-21-Lutetia.jpg, which was visited by Rosetta. They also tend to have a higher albedo than comets, due to comets being covered in large amounts of dark organic material. Neither would they be left with a crater the size of the one created at Tempel 1, due to being rock, whereas Tempel 1 (and all other comets) isn’t.
I posted a link to some sublimation experiments elsewhere, which showed the results of one of the samples that included organics (tholins): http://www.sciencedirect.com/science/article/pii/S0019103515005709. It’s behind a paywall, unfortunately, but the gist of the relevant finding is that this process left behind a rather hard surface layer. Further experiment will determine how hard.
So there is very good evidence indeed that comets are not rock. Just as there is very good evidence that the Moon is not made of Swiss cheese. And there isn’t a face on Mars.
What you are experiencing is called pareidolia. Fortunately, science doesn’t rely on pareidolia for its findings. What laypeople ‘believe’ due to their pareidolia is up to them; but it has no consequence regarding the outcome of what the evidence actually shows.

Hi SS.
Yes 67P looks like its made of “rock”, nobody disagrees with you, but to put it bluntly, “So What”? I have no wish to appear patronising, but a crystalline solid looks like a crystalline solid, whatever it is made of. “Rock” and the frozen volatile ices/interstellar dust mixture of 67P are both crystalline solids. Take a black and white photo of a very dirty snow drift and it will look remarkably like “rock” or 67P, as many bloggers before have previously provided photographic evidence for.

The surface of the “Cryocrete” on 67P is exposed to continuous harsh solar radiation which leads to sintering of the surface layers, a process also seen on Earth. Ice and snow develop a “harder” crust. The sintering process on 67P creates a porous structure with a “hard crust” in which interstellar dust is trapped within the matrix of the volatile’s ices. This process is well known and has been well studied here on Earth. I suspect the only difference on 67P is that because of the very low gravity and constant transport of volatile gases through the matrix due to the surface’s exposure to a vacuum, the ice lattice created is far more porous than on Earth. This allows more dust to be contained within it. The ices of these volatiles are largely colourless and transparent, so the resulting crystalline solid is the colour of the silicate based dust trapped within. That is, it is “rock” coloured. The constant flow of gas through the porous ice matrix also makes a large contribution to the process of mixing all the constituents into a visually, homogeneous solid.

In addition, as I suggested in an earlier post, the proposed process of accretion that created 67P, also generates considerable heat and thus thermal mixing of the constituents. Thus ensuring that mixing of the ingredients took place throughout the whole volume of the comet as it grew.

Erosion by thermal stress and the geological activity of a planetary object made of a crystalline solid, will also look the same. The peaks of mountains on Earth, look exactly the same as 67P in the summer when the snow melts. The same Physics, the same visual results. The evidence you ask for is all around you here on Earth in plain view. No human machinations are required at all.

One final thought. Are the rocks seen in various TV series, the original Star Trek for instance, actually rocks?
Of course not, we all know they are made of polystyrene, but the “disconnect” as you call it, is just the same. Two different materials that look like “rock”.

I hope this helps you in reconciling the difference between your perception and reality for 67P, which quite simply, is the answer to the “mind blowing disconnect” you mention. What one person perceives is reality to them, but it is not necessarily the same reality any other person perceives. Science is the attempt to create an ever more accurate, objective consensus of what reality actually is. It is NOT a dictatorial “assertion”.

The scientific method and the consensus it attempts to achieve, is fundamentally tied to the evolution of the collective knowledge created by human consciousness and hence should never be regarded as “absolute” or “the right answer”. Therefore you are perfectly entitled to disagree totally with all of the above, but I would humbly suggest you shouldn’t expect too many others to agree with you.

All good replies to my obstinately and repetitiously naïve post about the comet looking like rock, and yes, all pretty much the standard assertions that we’ve heard many times before. But here’s the deal, even knowing the things that the several reply posts above outline, I still choose to remain skeptical and first believe that the surface features actually are rock, for a number of reasons. First, I would like to point out that pretty much everyone seems to agree that the comet surface features actually DO look like rock. And if it obviously looks like rock, there needs to be a very good, detailed, direct evidence based understanding of what it is if it’s not rock, so much so that it completely overwhelms any possibility of it being rock, and we’re a long way away from that. Right now, no one knows what it is for sure – it might be this, it might be that, but no samples have been taken to confirm one way or the other. It’s assumed that it can’t be rock based on the current interpretations of the data and an ingrained, unshakable belief in what comets are. But belief based on evidence is a very tricky thing, and I guess I have too healthy a respect for the ability of science to not get it right based on evidence. People (yes, even scientists) find evidence for whatever they choose to believe, once they believe it, and tend to favor evidence that supports their belief and to ignore or discount the rest. And the commonality of all the replies to my post is that each person absolutely believes beyond any shadow of any doubt that the surface substance that obviously looks exactly like rock absolutely cannot be rock. And you all obviously feel you are on the firmest of scientific standing in your belief, and don’t seem to see any problem with this whatsoever. But is the evidence really overwhelming? Really? In my experience, evidence is rarely so iron clad as to be beyond dispute in any field, including science, and perhaps especially in science. Evidence is constantly being reinterpreted, new things discovered that overthrow previously held sacred cows, etc. Just look at the science of nutrition for an example of a wide variety of interpretations of conflicted data. And cosmology is no different. Still way too many unknowns, and your unquestioning confidence is based on a strong faith in commonly held generalities about comets, and you assume the details and specifics and realistic models and reality itself will eventually line up to your beliefs, which actually doesn’t seem all that scientific when you think about it.

@SS,
I’ll say it again; rock would be detected due to its totally different thermal inertia. It is hugely different from ice/dust/organics. It was easily detected by these same instruments at Steins. Why haven’t they detected it at 67P? Because it isn’t there!!!!!! No ifs, no buts, it just isn’t there.

What happened to this “rock” when a probe was smashed into it at Tempel 1? How big was the crater? What came out of the crater? Are any of the easily found answers to those questions consistent with it being rock?
Short answer; no, as a little bit of research would discover.

ian, ALL your posts present as “no ifs, no buts,” lol, regardless the issue. I’m afraid, though that I missed the paper where thermal inertia gave us a detailed, in depth analysis of exactly what the surface composition and characteristics are, but I’m sure you can provide us the link.

Now, from a body which HAS a rocky surface, asteroid Steins, measured by the same instrument:
“The thermal inertia of Steins was estimated to be in the range 450–850 J/(m2 s0.5 K)……..These values are rock-like, and are unlike the powdered-regolith surface of the Moon.”
And also very unlike those at 67P.

And of cousrse this doesn’t include the equally convinncing observations from Tempel 1. You wanted samples. Next best thing is to smash an impactor into a comet and see what size crater is left behind, and what is ejected from the impact crater (ice and dust).
So not a chance of it being rock.

SS: Agree that it looks like a hell of a crumbly substance. Remember how we saw an avalanche happening last June when the comet was at midsummer? That also suggests a crumbly substance. I would not think of a rock but something more porous. Why would midsummer heat (a little above 0 degrees Celsius) cause this substance to break up?

Hi Kamal,
This is neither an avalanche, nor indicative of a crumbly surface. The whole thing happened on a gravitationally low plain. It’s like an avalanche happening on a salt lake bed, in microgravity.

Since we are now getting back to orbits where we can expect lots more lovely detailed pictures, I have spent a few days trawling the NAVCAM and OSIRIS archives. I collected about 100 really amazing views. I have put some of them together in a video. In honour of the now absent Thomas I chose a “Rock” track as the musical accompaniment, so be warned. Unfortunately after YouTube has messed about with it, Holger’s lovely pin sharp pictures are anything but now.

Robin, this is too funny! That your post should follow right after mine. Your rock music I think is indeed appropriate as your video more than anything else I’ve seen shows overwhelming visual evidence that the surface is clearly and obviously common rock. Now, I know most here (including yourself) chose to not believe the obvious visual evidence due to other evidence, which is fine, and you could be right, but I seriously doubt it. Great collection of pics though, and artfully present.

Harvey, it’s rather amusing that everyone seems to agree that it looks just like rock (I guess everyone is experiencing the same “pareidolia”), that there’s a consensus, yet a consensus of visual interpretation of what it looks like in no way constitutes evidence, like this visual evidence simply can’t be scientific because we don’t believe in it. And yes, ice falls can have similar features, but there are rocky features on P67 that are not similar to them at all. And it’s the fact that there is such a wide variety of rocky formation features that I think should make one pause before so easily dismissing even the possibility. I guess it just concerns me when consensus agreement is so easily reached and so vigorously, unquestioningly defended based on still scant evidence when compared to an actual sample, or better yet, an on-comet, manned lab. But tell you what, if you know for certain that it’s not rock and all the surface features that look like rock have been formed by sublimation, ice and dust, I would think that, as with any other scientific field, you would have had to of analyzed exactly what it is and determined in detail exactly how the rocky surface features formed in order to declare absolute certainty. But obviously, whatever “evidence” that’s been gathered, or at least published, is no where near telling us that. And before you pull out the woo word yet again, ian, I’m not advocating EU. For all I know, this comet could be made of some mix of hard porous or light rock, dust and ice, and sublimation could still explain most of what we see action wise (is there anything besides the belief and strong faith in the solar system creation story that discounts this?). Anyway, just some thoughts.

Hi SS, Just for the record, I really don’t think it looks like rock. I just have never seen rock that black. It looks like coal to me, but I guess tar or pitch are about the same.. I can’t fathom that it can be ice (or rock for that matter) with just a paint coat thickness layer of black carbon matter.

@SS,
Sorry, no we don’t have to explain how the superficially “rocky” looking features formed to show that they aren’t rock. Without going through the thermal inertia data (other than to say that I accidently found the lower values I mentioned above: “The data are quantitatively
consistent with very low thermal inertia values, between 10-30 J K-1 m-2 s-1/2: from the 47th AAS meeting abstracts. On adsabs; “Search for regional variations of thermal and electrical properties of comet 67P/CG probed by MIRO/Rosetta”), and Tempel 1 results again, there is also the density of the comet, that is in no way consistent with it having any appreciable rocky component.
There are also polygons very suggestive of subsurface ice. These wouldn’t form beneath rock. On adsabs “Meter-scale thermal contraction crack polygons on comet 67P/Churyumov-Gerasimenko”
There are also estimates of tensile strengths of materials on the comet: http://www.aanda.org/articles/aa/abs/2015/11/aa26379-15/aa26379-15.html
“Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities.”
Then of course, we have the situation when this tensile strength is overcome, and boulders fall from cliffs, for instance. The fact that many of these are shown to be icy would further suggest that they are not rock: http://blogs.esa.int/rosetta/2016/01/13/exposed-ice-on-rosettas-comet-confirmed-as-water/
“The ice is associated with cliff walls and debris falls,………”

I could go on and on, but there is nothing that even remotely suggests that the surface is rock, and a lot of evidence that it isn’t.

SS: I find it very amusing that science is to be treated as though it has to be held up in a court of law. As to “manned labs”, would you care to travel to the ESA every time you want to make a posting on this blog?

The reason you see ‘rock’, is because that’s the world you live in. If you’d spent your entire life in an ice fall, and never seen rock, you’d see ice. I’ve spent a lot of time under water as a SCUBA diver and seen plenty of sea bed that looks like it; a sentient shark would probably see sea bed
🙂 partly because I have climbed, dived a lot, partly because I’m trained not to be fooled by appearance, I don’t ‘see rock’; I see something with a casual, vague similarity to rock – and to many other things.

Early in my career I spent a lot of time peering at SEM images of optical materials damaged by laser pulses. Looks pretty much like those too! Except the scale is a bit wrong……..

The image contains very little information about what it is made of. That comes almost entirely from other data, the density, thermal intertia, emitted gases, spectroscopy.

But the appearance alone is almost valueless, not quite. It tells you a bit about limiting mechanical properties.

We are trained to use *the evidence*; and these pictures contain very little evidence of what it is.

You continually allude to what is known as ‘confirmation bias’; seeing the data that confirms your theory, ignoring that which doesn’t, and it’s a very real problem. But in the end the process of publication and competition generally sorts it out. The old stuck in the mud just seeing what he wants to see is simply overtaken by the new guy who has got it right, because in the end if it’s right it will fit better, explain more etc.

We look *at the ensemble of the data* to reach our best guess at what it is, and most of that best guess is not guided by the pictures alone (though have to be consistent with them. They pose some constraints.)

Do I ‘know for certain it’s not rock’?
Well I know for certain most of the comet isn’t, the rest of the data is utterly incompatible with that. Multiple, completely independent types of data, different instruments, different physics, say it’s not.

Could there be *any* exposed rock? Yes, maybe, though there has been no clear evidence to support that. But you can’t say none, anywhere.

I return to my first point.
Imagine you’d lived in the Khumbu ice fall your whole life, and never seen a rock. (Yes, I know you can see rock from there, I’ve been there!) you’d say ‘wow, looks like home, it’s solid ice!’

You write, “… after looking back through many many photos of P67, the one thing that continues to be the most striking to me is that in literally every picture, no matter how close up or far away, every feature on the comet’s surface still looks either EXACTLY like dust, or EXACTLY like very hard rock …”

So, what does rock actually look like? A serious question! How do you know something is made of rock and not some other solid? Does rock even need to be solid? And what does dust look like on 67P? In all the “photographs” of 67P, there has never been a single picture of “surface” dust! We have seen smooth regions that may contain dust, but not dust per se! The highest resolution OSIRIS NAC images are still no better than tens of centimeters per pixel! And as you know, dust is so much finer than centimeter scale resolutions. So, what does rock actually look like?

From what you’ve said, you know what rock looks like. Now, imagine you’ve gone blind. What do you “see” when you look at a terrestrial mountain? I’m serious! Our past experience tells us that mountains are made of rock, yet you see no rock. Why is that? Are you blind? 😀

In past posts, I have discussed, in simple terms, the viability of the human eye as a useful scientific instrument. You will recall that a normal, healthy human eye can see EM wavelengths ranging from ~380 nm to ~710 nm – an incredibly narrow range compared to the full extent of the EM spectrum (i.e., picometers to kilometers)! In the previous paragraph, I asked you to imagine that you’ve gone blind. Truth is, we are blind! We do not have the capacity to really “see” … anything at all! Again, I ask, what does rock actually look like?

Let’s return to our terrestrial mountain example, and really open our eyes. From our vantage point, the base looks like it is made of hematite. Two hundred meters above that we see an exposure of dolomite. There also appears to be two different types of volcanic features. One looks like weathered basalt, the other, an intrusion made of hornblende. From here, it’s hard to tell if it’s iron or magnesium based. For completeness, we also observe some white stuff and patches of green things. So, what does rock actually look like? Can you really trust your eyes?

NAVCAM images are black and white (there’s no need for colour when navigating in space, or providing context for other scientific instruments). Yet this is where you start to see rocks and rocky comet morphology? VIRTIS records an incredible 860 bands of hyperspectral data from 250 nm to 5000 nm. Seems like a waste of time if we already know what rocks look like. Here’s a repost of the list of wavelengths some of Rosetta’s instruments operate at ….

Here’s another twist! NAVCAM and OSIRIS images do not show us rock. They show us comet geomorphology! In simple terms, geomorphology is the study of landforms, and the processes governing their origin and evolution – what you refer to as cliffs, boulders, peaks, ledges, striations, and stratifications. To be precise, these “landforms” are not rock – they may, however, be made of rock! But they could just as easily be made of ices covered by a thin nonvolatile organic-rich mantle. A cliff is only a cliff if a gravitational field leads to debris falls. Stratification implies deposition, but under what circumstances? Again, does gravity come into play? Is the apparent stratification the result of thermal cycling? It’s been said before – relying on Earth experiences and analogies may lead to spurious interpretations of what we think we see.

So, what does rock actually look like?

To me, science is great fun and I see all kinds of interconnections! In a recent post to originaljohn, I discuss the use of scale colours on VIRTIS temperature maps to address the ocular deficiencies associated with blindness in the IR portion of the EM spectrum. The following is a sample of additional deficits …

1) RSI – This instrument uses EM radiation to “see” the mass of an object. From the mass we can establish the objects bulk density. Asteroid 21 Lutetia has a bulk density of 3400 kg/m^3. Comet 67P has a bulk density of 533 kg/m^3. One is really made of rock. The other is really made of ices and dust (based on additional evidence from decades of observations).

2) MIRO – This instrument uses EM radiation to “see” water vapour and other volatiles emitted by the nucleus. Using water column density data acquired in September 2014, Biver et al (2015) were able to distinguish three unique regions within the coma. In general terms, the highest column density was observed emanating from the neck as a “concentrated” cone. Significant outgassing was also observed coming from the dayside, extending up to the terminator, while the nightside showed a very low column density that could be indicative of either low outgassing or backflow from the dayside. MIRO can also “see” subsurface temperatures down to a depth of 10+ cm. That’s like having submillimeter wavelength superhero vision!

3) VIRTIS – This instrument uses EM radiation to “see” infrared heat signatures, or more specifically, the surface temperature of the nucleus. The temperature maps produced by VIRTIS-M indicate that the comet is a cold celestial body (i.e., made of ices) with a low thermal inertia (i.e., covered in a fine insulating powder regolith). VIRTIS is also capable of mapping the mineralogical surface of an object. I personally don’t know any geologists that can actually “see” the mineral content of a surface just by looking at a photograph taken 20 km from the surface of a comet.

Furthermore, neither MIRO or VIRTIS identify any “rocky” outcroppings on the surface of 67P. There is no evidence of any surface feature (within the limits of the sensors resolution) exhibiting a thermal inertia consistent with that of rock. If the cliff faces were actually made of rock, why would they be evenly covered in the same fine powdery regolith? If the boulders were actually made of rock, why would they be evenly coated in the same fine powdery regolith? Remember, the whole surface of 67P shows a low thermal inertia and a reflectance spectra that is consistent with a compositionally homogeneous surface (Capaccioni et al (2015)) – Ref. Figure 1. And don’t forget the debris falls confirmed as water ice (linked above). The VIRTIS colour images show debris that exhibits a distinctive “icy” colour. So, no rock anywhere on the surface!

Shall I go on? As we can “see” (and this is just three examples), the human eye can see an inconsequentially small piece of the EM spectrum, but cannot “see” things like mass, water vapour, heat, mineral content, or rock (based on thermal inertia, amongst other things). So, how do you “know” for a fact that what you think is rock is actually rock? And in terms of optics, what else do you think we are missing out on …?

As shown above, the human eye is a rather useless sensing organ. Our eyes cannot sense the chemical signatures of poisons in plants. Our eyes cannot see a predator hiding in the dark. And our eyes cannot “see” rock on or in 67P. If 67P was made of rock, the scientific instruments on Rosetta would see it!

You ask, “So, which comes first, the chicken or the egg?”

Realistically? Neither! The single-celled organism came first! Over time, that critter evolved into something substantially larger and more complex – an organism that reproduces by laying eggs!

In the case of comets, amorphous solid water came first! Accretion is the evolutionary process that intimately mixes ASW with fine grained mineral dusts and other volatile and super-volatile molecules. Sadly, comets do not reproduce, though they do mate! You’ll recall that 67P is a contact binary?